1. Technical Field
The present invention relates to a CDMA base station handoff technique for reducing excessive handoffs in a mobile communication system in order to conserve forward link power and network processing resources.
2. Description of the Background Art
In conventional code division multiple access (CDMA) mobile communication systems, there may be an excessive number of handoffs per call as the mobile station moves, especially if the mobile station is located in a fading environment. Excessive handoffs of ten or greater may occur during a call, adversely impacting the forward link power and network processing resources.
IS-95 is a mobile to base station compatibility standard for dual-mode wide band spread spectrum. IS-95 is a direct-sequence CDMA scheme where users are differentiated on the basis of unique pseudorandom codes. Each signal consists of a different pseudorandom binary sequence that modulates the carrier and spreads the spectrum of the waveform.
According to version IS-95-A, upon initiating an originating call, a mobile station accesses a previously stored updated neighbor list of sectors, scans pilot signals transmitted from base stations on the neighbor list to determine pilot signal strength and locks onto a first strongest pilot signal. The mobile station then transmits a call initiation request to the base station which transmitted the pilot signal that the mobile station locked onto and the call is then set up on the respective traffic channel corresponding to the locked-on pilot signal. The traffic channel corresponding to this strongest pilot signal is designated as an active leg of the mobile station.
After the call is set up, the mobile station continues to scan the pilot signals of the neighbor list and determines pilot signal strength of all received pilot signals. The mobile station then generates a pilot strength measurement message (PSMM) which initially includes identification of the pilot signal first locked onto and the pilot signal strength of the pilot signal. Received pilot signals having pilot signal strength greater than predetermined threshold T_add are added to the PSMM as candidate pilot signals. T_add typically may be within the range of xe2x88x9212 dB to xe2x88x9214 dB. The mobile station transmits the PSMM to the corresponding base station of the existing mobile station active leg.
The base station sends the received PSMM to the mobile switching center which controls handoffs of mobile station active legs. If the PSMM includes only one existing mobile station active leg and plural candidate pilot signals, a traffic channel corresponding to a candidate pilot signal is established between the mobile station and the corresponding base station as a new mobile station active leg. In a CDMA system which allows three-way (maximum of three legs) handoff and the PSMM includes three existing mobile station active legs in addition to candidate pilot signals, the leg corresponding to a candidate pilot signal will be made active in place of a weakest existing mobile station active leg only if the pilot signal strength of the candidate pilot signal is greater than the pilot signal strength of the weakest active leg by at least a value of T_comp, which typically may be 3 dB.
Excessive handoffs such as ten or more per call may occur in a conventional mobile communication system as the mobile station moves within a fading environment. Even though a handoff may be preferred in the reverse link from the mobile station to the base station to maintain RF performance, excessive handoffs have negative impact on forward link power and network processing resources. Thus, there is a need to reduce excessive handoffs.
The present invention reduces the number of handoffs per call without impacting RF performance so that forward link power and network resources may be conserved. The base station screens candidate pilot signals from the PSMM generated by the mobile station and sends a revised PSMM to the mobile switching center. The mobile switching center controls handoffs based on the revised PSMM such that the number of handoffs may be reduced. In an alternative embodiment, the PSMM may be revised at the mobile switching center.